1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly to a method for accurately etching a SiN film and a BARC film.
2. Description of the Related Art
Conventionally, a method of dry-etching with a gas including CF4, CHF3, O2 and Ar has been proposed as a method for etching a BARC (bottom anti-reflective coating) film formed on an underlying SiN film.
FIGS. 12 to 14 are diagrams showing variations in cross-section of a gate structure having a step. In the case shown, the BARC film is formed at a predetermined portion in the gate structure by dry-etching with the aforementioned method.
In this gate structure, the step is formed, as shown in FIG. 12, by layering a polySi film 102, a WSi film 103, a SiN film 104 and a BARC film 105, in this order, on a substrate 100 at which a LOCOS (local oxidization of silicon) region 101 is formed. When the BARC film is etched with the aforementioned gas, a BARC residue 105xe2x80x2 at a step depression portion is generated in the etching process, as shown in FIG. 13.
The SiN film 104 is a hard mask for protecting the underlying films (i.e., the polySi film 102 and the WSi film 103). When over-etching is performed in order to remove the BARC residue 105xe2x80x2, an etching amount is 1.8 times as that of the BARC film. Consequently, as shown in FIG. 14, a portion of the etched SiN film 104xe2x80x2 that is protected by the BARC residue 105xe2x80x2 at the step depression portion is thicker than other parts of the SiN film 104xe2x80x2. Thus, the thickness of the SiN film 104xe2x80x2 becomes non-uniform. In this case, portions of the SiN film 104xe2x80x2 that are not protected by the BARC residue 105xe2x80x2 become excessively thin due to the etching.
In the circumstances described above, when further etching is performed on the SiN film 104xe2x80x2, because there are large variations in the thickness of the SiN film 104xe2x80x2, it is difficult to reliably perform subsequent etching of the underlying films with high accuracy, even if a selectivity ratio relative to the underlying films is adequate.
FIG. 15 is a diagram which schematically shows the form of a pattern in the case of formation by a usual hard mask etching process of the prior art, that is, a process of using etching gas of CHF3/O2/Ar with flow rates of 20/5/400 (SCCM) in a magnetron RIE (reactive ion etching) device, and then performing etching of an SiN/SiO2 film 111, which is a hard mask for protecting a polySi film 110, under etching conditions of: substrate RF power=500 W; pressure=40 mTorr; and electrode temperature=40xc2x0 C.
As is shown in FIG. 15, the SiN film is etched substantially vertically at high-density pattern portions having a high degree of integration, such as, for example, cell portions for a DRAM. However, low-density pattern portions, peripheral portions and the like, which gas that tends to cause deposition can easily enter, become tapered due to the effect of deposition material. Thus, there is a problem in that lateral dimensions vary in relation to density variations of the pattern.
A main object of the present invention is to provide a semiconductor device fabrication method which, at a time of etching a BARC film, can increase an etching selectivity ratio of underlying films and enables optimal etching regardless of a degree of density variation of a pattern, and which, at a time of hard mask etching of a SiN film or the like, enables optimal etching regardless of the degree of density variation of the pattern.
According to a first aspect of the present invention, in a semiconductor device fabrication method which includes etching a BARC film disposed with an underlying SiN or polySi film, the etching of the BARC film includes dry-etching the BARC film with etching gas in which at least O2, Cl2 and He are mixed with predetermined flow volume ratios; and when an etching selectivity ratio of the BARC film relative to the underlying film is to be made larger, increasing the flow volume ratio of O2 relative to Cl2.
According to a second aspect of the present invention, in a semiconductor device fabrication method which includes etching a BARC film disposed with an underlying SiN or polySi film, the etching of the BARC film includes dry-etching the BARC film with etching gas in which at least O2, Cl2 and He are mixed with predetermined flow volume ratios; and when an etching selectivity ratio of the BARC film relative to the underlying film is to be made larger, reducing ion energy at the time of etching.
According to a third aspect of the present invention, in a semiconductor device fabrication method which includes etching a BARC film disposed with an underlying SiN or polySi film, the step of etching the BARC film includes dry-etching the BARC film with etching gas in which at least O2, Cl2 and He are mixed with predetermined flow volume ratios; and setting the flow volume ratio of O2 relative to Cl2 in the etching gas to at least 1.
According to a fourth aspect of the present invention, in a semiconductor device fabrication method which includes etching a SiN film, the etching of the SiN film includes: at the time of etching the SiN film, using etching gas in which HBr, CF4 and He are mixed in predetermined flow volume ratios; and setting ion energy at the time of etching in a range from 13 eV to 30 eV.